In recent years, liquid crystal display devices are rapidly spreading, replacing cathode ray tubes (CRTs). The liquid crystal display devices have advantages of low energy consumption, thin profile, light weight, and the like, and are widely used as display panels for television receivers, personal computers, mobile phones, and the like.
In driving a liquid crystal display device, if a direct current voltage (DC voltage) is applied to liquid crystal molecules over a long period of time, characteristics thereof are deteriorated. Therefore, in order to avoid this problem, a polarity inversion driving method is generally employed such that the polarity of a voltage applied to the liquid crystal molecules is periodically reversed. One of the conventional polarity inversion driving methods is a line inversion driving method. In this method, the polarity of the voltage that is applied to the liquid crystal is reversed for respective adjacent bus lines. That is, in the first frame, all of the pixels on odd-numbered bus lines are applied with a positive polarity voltage, and all of the pixels on even-numbered bus lines are applied with a negative polarity voltage. Thereafter, in the second frame, all of the pixels on the odd-numbered bus lines are applied with a negative polarity data voltage, and all of the pixels on the even-numbered bus lines are applied with a positive polarity data voltage.
Recently, a driving method for a display device that can achieve a reduction in power consumption has been disclosed. With this method, by having an idle period in which all scanning signal lines are in a non-scanning state, the power consumption for driving a liquid crystal display device is reduced. For example, Patent Document 1 discloses a configuration of a display device that has a writing/scanning period and a non-writing/scanning period (idle period). In the writing/scanning period, the display portion is applied with voltages by line-scanning, and data writing is performed, and in the non-writing/scanning period (idle period), data is not written. In this configuration, the display device operates in a normal operation mode during the writing/scanning period, and operates in a power saving operation mode that uses less power than the normal operation mode during at least part of the non-writing/scanning period.
When the driving method with the idle period is combined with the above-mentioned line inversion driving method, if no source voltage is outputted from the data signal lines, flickering would occur. This is because a change in brightness is caused by a parasitic capacitance Csd between each data signal line and each drain electrode when the driving period and the idle period are switched over to each other.
In order to solve this problem, when the driving method with the idle period is combined with the line inversion driving method, the frequency of the source voltage during the idle period was set low. FIG. 6 shows waveforms of various signals in this case.
As shown in FIG. 6, during the normal driving period, the frequency of the output of the source voltage is set to high, and during the idle period, the frequency of the output of the source voltage is set to low. As a result, while source voltages are outputted during the idle period, because the frequency thereof is set to low, a reduction in power consumption can be achieved, and it is also possible to prevent the degradation in display quality caused by flickering.